In order to transmit and radiate image light to a light receiving body having a spectral waveform characteristic, such as a photographic photosensitive material, it is well known to use an optical system provided with a reflecting apparatus for reflecting image light from an object. In such an optical system, even if light of a wavelength to which a light receiving body is slightly sensitive is radiated and transmitted to the light receiving body, the latter does not become sensitized at all. Therefore, the reflection and transmission of light of such a wavelength is meaningless in sensitizing the light receiving body. Moreover, such light acts as interfering light, so it should be absorbed.
To avoid the aforementioned problem, there has heretofore been used a reflecting apparatus for an optical system in which a colored glass filter is provided, whereby only image rays having a spectral sensitivity characteristic approximate to that of the light receiving body are selectively reflected.
Generally speaking, however, a colored filter has a property such that light rays having a spectral wavelength characteristic approximately equal to that for selective reflection are transmitted, and light having other spectral wavelength characteristics are absorbed. In the course of selecting a spectral reflection characteristic, therefore, the loss in the quantity of light is large and a portion of the transmitted light causes what is known as a flare phenomenon. It is therefore necessary to eliminate such transmitted light.
It has also been proposed, for example in U.S. Pat. No. 4,132,477, to substitute for the aforementioned colored filter a dichroic mirror or interference film layer or layers formed on a transparent base substrate from substances having different refractive indices. In such a film layer or layers, however, since the spectral wavelength characteristic of light to be selectively reflected and that of light to be selectively transmitted are complementary, the quantity of selectively reflected light is large. Moreover, if the spectral characteristic of the film layer is well matched to that of the light receiving body, the flare phenomenon caused by selectively transmitted light is eliminated, but it is difficult to precisely match the two spectral characteristics, so the transmitted light acts as an interfering light unless it is cut off completely.
To solve this problem, in the above-cited U.S. Pat. No. 4,132,477 and as illustrated in FIGS. 8 and 9 thereof, a transparent glass or plastic body is used as a substrate, and having a face confronting a converging lens, on which is formed a dichroic mirror. The back of the substrate is processed so as to be light-diffusing. Furthermore, a light absorbing plate is provided on the extension of the optical axis behind the substrate. The casing for these components is tightly shielded at the back thereof. These fabrication requirements are troublesome, leading to increased cost.